A nuclear fuel assembly consists of a matrix of parallel rods containing fissionable fuel and/or water coolant flow. The fuel rods are sealed at the top and bottom ends by end plugs welded to the fuel rods. These parallel rods are held at a fixed lateral spacing by spacer meshes located intermittently along the length of the fuel assembly. The matrix of fuel rods is supported at the bottom by a lower tie plate which provides lateral guidance for the fuel rod lower end plugs, and which includes flow holes providing an inlet for coolant flow into the fuel assembly. Similarly, the top end of the rod matrix is covered by an upper tie plate which restrains the fuel rod upper end plugs laterally and which includes flow holes providing an exit for coolant flow out of the fuel assembly. Each fuel bundle is enclosed within an open-ended channel also extending between the upper and lower tie plates.
One or more of the water and/or fuel rods may be used as structural members which rigidly fasten by some means to both the lower and upper tie plates for the purpose of lifting the assembly and maintaining a fixed distance between the lower and upper tie plates. Other fuel and/or water rods in the assembly not used as structural members are either restrained by a threaded or other releasable joint to the lower tie plate or are prevented from lifting off the lower tie plate by the upper tie plate or by expansion springs. Since reduced diameter sections of the upper end plugs typically extend through the upper tie plate holes, the fuel rods are free to expand in length until the shoulder on the upper end plug contacts the upper tie plate or until the expansion springs are fully compressed.
FIG. 1 illustrates in simplified form, components of a conventional fuel bundle assembly for a boiling water nuclear reactor. Specifically, the fuel bundle assembly 10 includes a plurality of fuel rods 12 extending in parallel relationship between an upper tie plate 14 and lower tie plate 16. The fuel rods 12 are typically arranged in a square array best understood from FIG. 1A. Centrally located within the fuel rod array is a water rod 18 also extending between the upper and lower tie plates. The lower tie plate 16 as shown is an integral part of a transition piece TP which includes at its lower end a coolant inlet 20. The entire fuel bundle is enclosed within an open-ended channel 22 of substantially square cross section (see also FIG. 1A.)
The fuel rods typically are provided at upper and lower ends with end plugs 24, 26. The lower end plugs 26 are seated within apertures 28 formed in the lower tie plate 16. The lower tie plate 16 is also formed with a plurality of coolant flow apertures 30. Similarly, the upper end plugs 24 of the fuel rods 12 are seated within apertures 32 formed in the upper tie plate 14, and the tie plate 14 is provided with coolant flow openings 34. The arrangement of apertures for receiving fuel rod end plugs and coolant flow openings is apparent from the grid-like construction of the upper tie plate 14 as shown in FIG. 1A, a similar grid-like arrangement being provided on the lower tie plate 16.
In this prior arrangement, water rod 18 is the load bearing structure by which the fuel bundle assembly may be lifted via handle 46. More specifically, the water rod 18 is threaded into the lower tie plate 16 by means of a threaded stud or end plug 36, while the upper end of the water rod 18 is threadably secured to the upper tie plate 14 by means of a threaded stud 38 and nut 40. An arrangement generally of this type is described in U.S. Pat. No. 5,327,471.
The water rod is also formed at its lower end with a plurality of side entry coolant openings 42, and at its upper end with a plurality of side exit apertures 44. Other coolant opening configurations, such as axially oriented openings are also common (see the '471 patent).
It is also noted here that the upper tie plate 14 may support the channel 22 by means of upper tie plate extensions 48, 50 which bear directly on corner gussets 52, 54 welded at the upper end of the channel. A channel guide 56 is also secured at one end of the channel and, in conventional fashion, serves to space adjacent fuel bundle assemblies within the reactor core. With this arrangement, while the water rod 18 is the load bearing number, the upper tie plate 14 is connected to the channel 22 so that the channel is lifted out of the core along with the fuel bundle. The upper tie plate can be disconnected from the channel, however, by removal of bolt 58, permitting the channel itself to be lifted off the fuel bundle assembly.
Expansion springs (not shown) may be used to push downward on the upper end plug shoulders 59, biasing the fuel rods downwardly upon the lower tie plate 16. Such springs extend between the shoulders 59 and the underside of tie plate 14 in the usual fashion, so that the fuel rods are free to expand in length until the springs are compressed fully between the upper end plug shoulders and the upper tie plate. With the use of expansion springs, the rods used as structural members (the water rod 18 in the example provided) are typically loaded in tension and the remainder of the rods with expansion springs are loaded in compression.
In designing a nuclear fuel assembly, one of the limiting constraints for very high exposure capabilities is the pressure built up in the fuel rods due to fission gas release. Also, the differential irradiation growth of the fuel rods and water rods becomes more significant at high exposures, requiring very long end plug extensions which are guided laterally by bosses in current upper tie plate designs. These long end plug extensions reduce the length available for the fuel rod plenum used to accommodate the fission gas release. The upper tie plate and upper end plugs designs currently used require complex machining, and these components, as well as the expansion springs, are costly.